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Getting better roof insulation

erikdavitt | Posted in Plans Review on

I am 8 months into owning my first house and what a disaster it has been. The house is located in NH, climate zone 6. It was built in 2009 and my wife and I are the first people to live in the house. This winter has been especially difficult. We have had many issues with ice dams and leaking through the Soffit areas into the external walls.

The house is about 2000 sq feet and is an energy hog. So far this winter I have been through multiple chords of wood and propane tanks… Otherwise the house is a fairly simple design, I would say it is ranch style with a large dormer on the front. It is a one story house with a walk out basement which is made to be finished however it is not finished at this time. The kitchen, living room and family room have cathedral ceilings. The other bedrooms have unused attic space above.

With our ice dam issues over the winter we have had to remove drywall off of the cathedral ceilings due to water damage. The builder only put 2 ft of sheet metal on the eaves to protect from ice damage. Also turns out the cathedral ceilings have R38 fiberglass insulation and there is no roof venting whatsoever. The builder also made no effort in air sealing the house.

If I had known how important these things were I probably would not have bought this house. I knew so little back then about building science, but have been learning a lot over the last 8 months.

This summer I am looking help fix some of the issues the house is having. I have the opportunity to attach the issue from both sides. As drywall has started to come down and the house will need a new roof so adequate iceguard can be placed. During this process of renovating I would like to boost the energy efficiency of the house and boost its insulation and air sealing.

I have considered different options for getting there and am looking for options/advice of others.

Option 1:
I have considered foam over the roof sheathing on the outside. To hit an r-value of 60 in my climate zone I would need around 50% of my insulation to be on the exterior. So around an r-value of 30.

My thoughts for this design are:

Option 1:
1) expose sheathing.
2) apply water/icegaurd more for air sealing.
3) Multiple layers of staggered foam.
4) strap foam with 2x4s to create 2 inch ventilation channel.
5) apply new roof sheathing with water/icegaurd.

Then on the inside would have around r30 fiberglass batts.

-The difficulties I find with this method are that the whole house is not cathedral ceilings and the roof is in one plane. Given half my house is in attic space which is difficult to access I would hate to turn it into conditioned space. Second in my area I am finding it very hard for anybody willing to put down that much foam board.

Option 2:

For this option I was thinking of attacking the issue solely from the inside.

For spaces over the attic I would try to air seal the attic as best that could be done. Then from there I would look to put in blown cellulose. At least enough to get me an r-value of 60. I would then also add soffit vents, as at this time the house does not have any.

The cathedral ceilings I think are a little more complicated. The rafters are 2×12. Starting from the roof sheathing my thoughts would be:

1) 2 inch ventilation channel with the addition to soffit vents
2) This ventilation channel would be created by using cut foam board about 4 inches thick (I’m thinking EPS as it will be more exposed to cold.
3) Then 5 inches of closed cell spray foam to bring insulation flush with the rafters
4) To stop thermal bridging I could use another 4 inches of foam board
5) From there I would place strapping and then drywall.

My only problems with this method is I don’t know if it is wise or would work. And my other question is with foam board on the interior how do light fixtures hang securely?

Any thoughts ideas or opinions would be greatly appreciated.

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Replies

  1. Expert Member
    Peter Yost | | #1

    Hi Erik -

    I am sorry that you have a new home that is not even built to code. I am in southern VT and I spend quite a bit of time in NH working to assess/fix old and new homes with energy and related moisture problems. Equally sad is that in 8 months of online study you probably know more building science than many practicing building professionals.

    To start: ice dams are almost always a combination of conductive (lack of insulation) and convective (air leak) heat loss. You say your home is a ranch-style and maybe the roof pitch is low (less than 5:12) and that makes it more difficult to get adequate insulation and air sealing at the eaves.

    In the two options you describe, each has advantages/disadvantages:

    1. Continuous insulation on the exterior of an assembly warms everything to the interior (reducing the potential for wintertime condensation in the roof assembly) and makes it much easier to get full depth of insulation and continuous air sealing in your entire roof.

    A key consideration with this approach is: connecting your roof and wall air barrier at the eaves.

    Good GBA resource here is: https://www.greenbuildingadvisor.com/article/solving-an-ice-dam-problem-with-exterior-rigid-foam.

    2. Option 2 seems less disruptive, focusing on interior work. But what you describe makes achieving a continuous air control layer more challenging.

    And don't forget: soffit ventilation only works if it is connected to ridge venting; you probably know this but you did not mention it in your description of the two options.

    Peter

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